This invention relates to a ridged waveguide-type semiconductor laser device having a reduced threshold current as well as other improved and useful properties.
A conventional ridged waveguide-type semiconductor laser devices typically has a structure as illustrated in FIG. 4 and is produced through a manufacturing process as described below.
First, an n-InP clad layer 2, an active layer 3, an antimeltback layer 4, a p-InP clad layer 5, a p-GaInAsP cap layer 6 are sequentially grown in the described order on an n-InP substrate 1 to form a multilayered structure and then an SiO.sub.2 insulation layer is deposited on the top,
Then, an etching mask is prepared by a photolithography technique to cover specific areas of the multilayered structure and the covered multilayered structure is etched as deep as the antimeltback layer 4 to produce a pair of grooves cut into said multilayered structure.
After removing the etching mask, an insulation layer 7 is formed thereon and again a photolithography technique is used to cut a window for ohmic contact of a p-side electrode.
Finally, a p-side electrode 8 and an n-side electrode 9 are formed respectively on the upper surface of the multilayered structure and the lower surface of the substrate 1.
A semiconductor laser device as described above shows different refractive indexes relative to the active layer at the ridge portion located at the center of the upper surface and at the lateral grooves on the sides of the ridge portion. Consequently, it is normally provided with a waveguide designed on the basis of the difference of refractive index. Therefore, such a semiconductor laser device is normally called a "ridged" waveguide-type semiconductor laser device.
Since a ridged waveguide-type semiconductor laser device can be manufactured by way of a single epitaxial growth process, it is superior to semiconductor laser devices having a different structure in terms of yield, cost and mass production.
A ridged waveguide-type semiconductor laser device having the following characteristics is described by A. M. Rashid, IEEE J. Lightwave Tech. Vol. 6, 1988, pp. 25-29: threshold current level Ith=25.about.30 mA, optical output Pmax=30 mW, quantum efficiency .eta.o =20.about.25% (oscillation wavelength.lambda.=1.3 .mu.m), angle of horizontal beams relative to the substrate .THETA..sub..parallel. =16.degree. and angle of vertical beams relative to the substrate .THETA..sub..perp. =30.degree..
A known ridged waveguide-type semiconductor laser device is, however, accompanied by technological problems such as:
(1) Low current confinement effect and a high threshold current level.
(2) Large .THETA..sub..perp. /.THETA..sub..parallel. ratio due to the beam angles of .THETA..sub..perp. =30.degree. and .THETA..sub..parallel. 16.degree..
(3) The ridge stripe width cannot be made narrow since a window should be formed for ohmic contact within the ridged portion.
(4) Poor thermal characteristics because of poor heat conduction capability.